What is a OG Protection Relay and How to Choose One?
Medium voltage (MV) protection relays, a cornerstone of safety in electrical systems, are used to protect circuit elements in medium voltage networks against faults. Protection systems are of paramount importance, especially in the energy supply chain from generation to consumption, as a potential fault can disrupt the balance of the entire system.
Protection relays used in medium voltage systems, thanks to their microprocessor-based structure, analyze data received from measuring transformers and send a tripping signal to circuit breakers when they detect an anomaly in the system. In this way, the fault is quickly isolated, and the rest of the system continues to operate.
What is the function of an MV Protection Relay?
A protection relay continuously monitors the current and voltage values in the system. When conditions such as overcurrent, ground fault, voltage drop, or phase imbalance are detected, the relay evaluates the situation according to predefined set values and intervenes in the circuit breaker if necessary.
An overcurrent relay instantly detects the situation when the current in the circuit exceeds the limits and opens the circuit breaker, preventing damage to the devices in the system. Relays perform this intervention in milliseconds. These relays are equipped with different functions to protect equipment such as transformers, motors, generators, and lines.
Considerations When Selecting an MV Protection Relay
The selection of an MV protection relay should be based not only on the technical specifications of the system, but also on the needs of the facility, protection scenarios, and selectivity requirements.
Selection Criteria:
Electrical protection equipment (transformer, motor, line, etc.)
ANSI protection function codes
Relay communication protocol (Modbus, IEC61850, etc.)
Fast opening/closing time
User interface and software support
Set value adjustment accuracy
Protection functions are defined according to ANSI standards, and the functions each relay has are specified in its technical documentation. For example, while the MV line protection relay must have features such as directional overcurrent (67) and earth leakage protection (51N); if motor protection is to be performed, the thermal overcurrent (49) function becomes critical.
Most Commonly Used ANSI Protection Functions
The most commonly used ANSI codes and their corresponding meanings in medium-voltage transmission and distribution systems are listed below:
27 Undervoltage Protection
32 Directional Power Protection
46 Phase Imbalance (Negative Current)
49 Thermal Overcurrent Protection
50 Inrush Overcurrent Protection
50N Inrush Ground Fault Protection
51 Time-Delayed Overcurrent Protection
51N Timed Ground Fault Protection
59 Overvoltage Protection
67 Directional Overcurrent Protection
67N Directional Ground Fault Protection
81U Low Frequency Protection
81O High Frequency Protection
87T Transformer Differential Protection
87L Line Differential Protection
The correct relay selection should be evaluated according to these functions.
What is an Overcurrent Protection Relay?
In electrical installations, overcurrent protection relays play a critical role in ensuring system safety. Overcurrents can occur as a result of short circuits, sudden overloads, or various system failures. A properly configured overcurrent relay can protect electrical equipment from serious damage in these situations.
Different models, such as motor overcurrent relays, high current relays, and time-delay relays, are chosen according to the application type. The set values of these products must be programmed according to the ratios and operating characteristics of the measuring transformers in the field.
Choosing the correct protection relay plays a critical role in medium-voltage systems, both in terms of equipment safety and the protection of field personnel. These systems, which provide quick and selective intervention in case of a fault, offer significant advantages in terms of energy continuity and equipment health.
A medium-voltage protection relay with the correct functions and programmed with appropriate set values ensures the safe operation of your system for years to come. This precaution against events such as overcurrent, ground faults, and voltage surges prevents major failures and costs in the long run.